PresShell::EventHandler shouldn't be used to dispatch untrusted event.
However, it checks whether the given event is trusted or untrusted somewhere
and that makes the code harder to understand. So, it should check each event
only with MOZ_ASSERT() or MOZ_DIAGNOSTIC_ASSERT() instead. Then,
developers can trust the event is always a trusted event.

PresShell::EventHandler::PrepareToDispatchEvent() checked whether the
given event is a trusted event or an untrusted event, butPresShell::EventHandler::PrepareToDispatchOntextMenuEvent() didn't so.
However, now, both of them don't need to check it. Therefore, we can merge
them.

In normal cases, PresShell::EventHandler won't receive untrusted event.
However, only nsIDOMWindowUtils.dispatchDOMEventViaPresShell() may send
untrusted event if its aTrusted is false. Currently, this is not used by
chrome so that we don't need to keep supporting it for dispatching untrusted
events.